This invention relates to cathode-ray tubes of the type having a shadow mask mounted in spaced relation to a cathodoluminescent screen, and particularly to a temperature compensating means for the shadow masks within such tubes.
Shadow mask type cathode-ray tubes usually include a screen of red, green and blue emitting phosphor lines or dots, electron gun means for exciting the screen, and a shadow mask interposed between the gun means and the screen. The shadow mask is a thin multiapertured sheet of metal precisely disposed adjacent the screen so that the mask apertures are systematically related to the phosphor lines or dots.
In shadow mask type cathode-ray tubes, the accuracy with which the electron beams strike the individual elemental screen areas depends to a great degree upon the accuracy with which the mask apertures are aligned with the elemental screen areas during operation of the tube. Thus, should the mask expand by reason of thermal effects occasioned by the impact thereon of the electron beam or beams, the resulting misalignment of the mask apertures and elemental color areas may cause at least some of the beam electrons to impinge upon elemental color areas other than the ones upon which they were intended to impinge.
Several early methods or means were proposed for compensating for thermal expansion of the mask by causing the mask to move (axially) toward the screen as it expands, outwardly to maintain the desired alignment of the mask apertures and elemental screen areas. U.S. Pat. No. 2,795,719, issued to Morrell on June 11, 1957, proposed movably mounting the mask within the envelope by means of three carriages attached to the periphery of the mask and sliding on inclined tracks mounted on the envelope. U.S. Pat. No. 2,795,718; issued to van Hekken on June 11, 1957, proposed the use of a multiplicity of flexible hinges connecting the masking member with a supporting frame, or a pivotal bell crank having arms slidably engaging the mask. These compensating means were designed primarily for use with circular masks in round tubes of moderate size and moderate deflection angle.
A later thermal compensating means was disclosed in U.S. Pat. No. 3,803,436, issued to Morrell on Apr. 9, 1974. In this patent, either a leaf spring of special shape or a combination of a box spring and a bimetallic element connected between a stud imbedded in the tube envelope and the mask is shown. Presently, most shadow mask type cathode-ray tubes use a bimetallic spring or a bimetallic element between a spring and the mask to provide for temperature compensation.
Although the foregoing patents do provide means for compensating for long-term heating of a shadow mask, they do not address the problems associated with short-term heating. Short-term heating causes doming and thus distortion of the mask shape since all portions of the mask and its associated support frame are not of uniform temperature. There are two principal methods which have been used in commercial tubes to address the short-term or doming problem. The method most widely used consists of varying the number and position of the mask-to-frame welds, such as disclosed in U.S. Pat. No. 3,368,098, issued to Demmy on Feb. 6, 1968. In another method, such as disclosed in U.S. Pat. No. 3,703,401, issued to Deal on Dec. 28, 1970, a shadow mask is coated with a material such as carbon to alter its thermal radiation characteristics.
Yet another means of compensating for doming is disclosed in U.S. Pat. No. 3,872,345, issued to Yamazaki et al. on Mar. 18, 1975. This patent shows the use of a corrugated mask surface having contiguous concave and convex areas over the apertured portion of the mask.
Although the last-named patents provide individual solutions to the long and short-term heating problems, there still is a need for additional solutions that provide the required mask compensations at potentially lower costs.